This invention relates to improvements in the smelting of non-ferrous metals, particularly copper and nickel. More specifically, the invention relates to increasing the processing capacity of reverberatory furnaces, elevating the efficiency of energy utilization, and facilitation of exhaust gas treatment for the purposes of preventing pollution.
As one example of smelting of non-ferrous metals, copper smelting may be considered. A copper smelting process widely practiced at present may be broadly divided into two stages, i.e., a smelting stage wherein a sulfide ore (which is hereinafter used collectively as a generic term for starting materials charged into reverberatory furnace as copper smelting starting materials such as ores and concentrates) is heated to be melted and oxidized thereby to give an anode of a copper content of at least 99 percent, and an electrolytic refining stage wherein this anode is electrolyzed to produce electrolytic copper. The smelting stage further comprises a smelting step wherein the ore is melted to concentrate the copper value into matte, a copper producing step wherein the matte is oxidized and thereby converted into blister copper, and a refining step wherein the blister copper is refined and cast into an anode, these steps being carried out successively and consecutively in the stated order.
This invention provides a novel method of operating a reverberatory furnace used in the ore smelting step of the above stated steps.
In general, reverberatory furnaces have been used from early times for smelting copper ores, and, even today, a large number of these furnaces are in actual operation at various places in the world because of their advantageous features such as their suitability for processing large quantities of ores, the great ranges of kinds and grades of ores which they can process, and the relative simplicity of the manipulative procedure for their operation. However, this method of using a reverberatory furnace is accompanied by certain problems such as the following.
(1) Because of the mechanism of the reverberatory furnace operation wherein combustion burners are operated in an open space and radiation heat and convection heat are chiefly utilized, the fuel consumption rate is high, whereby this type of furnace is not necessarily advantageous in this age of high energy cost.
(2) Because of the flat construction of the furnace, the processing capacity per unit area of the furnace bath, that is, the furnace bath efficiency, is low, and the overall efficiency is poor.
(3) Since the fuel is consumed in a large quantity, the gases generated from the ore are diluted by the combustion gases, and the SO.sub.2 concentration of the exhaust gases from the furnace is low. Moreover, since the gas quantity is large, the cost of gas treatment for prevention of pollution increases.
(4) The grade of the copper within the matte is dependent chiefly on the composition of the starting material ore, and little room is left for control thereof. For example, when ordinary ore of a copper content of the order of 28 percent is processed by the green charge method (i.e., a method wherein a starting material concentrate is charged into the furnace without being roasted or calcined and with a moisture content of the order of 7 to 8 percent), the copper content within the matte (i.e., matte grade) is of a low value of the order of 36 to 38 percent, which accordingly gives rise to an increase in the load for matte treatment in the converter.
When the calcine charge method (i.e., a method wherein the starting material ore is calcined beforehand, a portion of the sulfur being removed, and is then charged into the furnace) is used, the matte grade becomes a value of the order of 45 to 55 percent. However, if the matte grade is raised above this range, the copper content within the slag will abruptly increase to result in copper loss. Another difficulty is that the calcine charge method requires large-scale equipment for pretreatment. Furthermore, in the reverberatory furnace, the only operation carried out is melting of the ore to separate it into matte and slag, and oxidation of the matte is not actively carried out. For this reason, heat of oxidation cannot be utilized for melting the ore, and this is one cause of the high fuel consumption rate of a reverberatory furnace.
For the purpose of overcoming these difficulties, various attempts to improve the construction and control procedure of reverberatory furnaces have been made. For example, in the process disclosed in U.S. Pat. No. 3,436,068 (Japanese Pat. No. 18042/1969), a water-cooled lance is used to blow air into the melt within a green charge type reverberatory furnace thereby to cause the melt to be splashed onto the ore bank and promote the rate of melting of the ore. In the process disclosed in U.S. Pat. No. 3,664,828, the ore is melted in a furnace of the shape of a reverberatory furnace, a lance being inserted therethrough at the same time to blow air into the melt and thereby to convert the matte into blister copper, and the above mentioned two reactions of the smelting step and the copper producing step are caused to progress in the same furnace.
However, in the process disclosed in U.S. Pat. No. 3,436,068, since the slag layer in a reverberatory furnace is ordinarily of a thickness of 40 cm. or more, the air blown in ordinarily does not reach the matte layer, whereby effective oxidation thereof is not carried out. Even if air is blown to the matte layer, a great amount of splashing will occur and mechanically carry ore particles and matte particles into the slag layer to become suspended therein. These particles are thus not separated, whereby the copper loss tends to increase. In addition, a water-cooled lance is easily clogged, and, if the cooling water should leak and contact the matte within the furnace, there would be the danger of an explosion. Thus, this process entails complications in handling, which cannot be avoided even if a cooling medium other than water is used.
The process disclosed in U.S. Pat. No. 3,664,828 is characterized in that the interior of the furnace is provided with zones for carrying out three different functions, namely, a melting zone, a converting zone, and a settling zone. Because of the mutual interaction of these zones, not only is the manipulative procedure for controlling the process extremely difficult, but, in actual operation, demarcations of these zones with different functions become indistinct, whereby the distribution of the belt becomes obscure and the control of the respective zones becomes difficult. Furthermore, in order to carry out slag-matte separation by agitation of the melt by blowing air thereinto through the lance and settling, a larger settling time becomes necessary, and for this reason the settling zone must be widened, or a thick slag layer must be formed. Widening of the settling zone means an increase in the bath area of the furnace, which will give rise to an increase in heat loss. Forming a thick slag layer necessitates an elevation of the pressure of the blown-in-air, which, as a natural result, will lead to a vigorous flow movement of the melt, whereby the resulting rapid wear of the refractory materials of the furnace wall will constitute an economically great load. Furthermore, vigorous agitation naturally will increase the quantities of the matte and starting material ore suspended in the slag, and the copper content in the slag will become high even after passage through the separation region, which will cause an increase in copper loss.
In this connection, there has also been proposed a so-called continuous copper production process as disclosed in U.S. Pat. No. 3,890,139 (Japanese Pat. No. 43015/1976) which was assigned to the assignee of this application. This process comprises: a first process step in which a metal sulfide ore as a starting material is blended with a flux and/or fuel and continuously blown into the melt by pressurized air to be melted; a second process step in which all the product of the first process is charged into a separation furnace and thereby separated into matte and slag, which are taken out; and a third process step in which the matte and an appropriate proportion of flux are continuously charged into a copper producing furnace, and white matte or blister copper and converting furnace slag are produced. These process steps are so arranged that they can be controlled independently of one another, and means are provided to convey the melts continuously between the furnaces in which these process steps are carried out. In the melting of the starting material ore in the above stated first process step, the ore is melted by blowing it into the melt in a smelting furnace with the object of increasing the melting efficiency. It is also suggested in the patent that a reverberatory furnace can be used for this smelting furnace.
However, the above described first process step is still accompanied by a number of problems, the most difficult of which are as follows.
(a). Since, in the above described process, an improvement in the melting efficiency is sought by blowing the starting materials continuously into the hot melt, the melt is vigorously agitated, whereby the corrosion of the furnace wall bricks is severe. For this reason, measures such as installing jackets in the bricks for cooling are being resorted to, but the heat efficiency decreases, and even with such measures, it is necessary to stop the operation periodically and to make repairs. Such damage is unavoidable as long as the melt contacts the furnace surface.
(b). In the above described process, all of the starting material must be subjected to a pretreatment such as drying or calcining thereby to remove moisture and a portion of the sulfur content and to impart fluidity to the material before it is fed. For this reason, a pretreatment step becomes necessary, and large-scale equipment is required. However, a smelting furnace cannot be easily made large since, as mentioned above, it must be periodically repaired, and for ordinary operation, it becomes necessary to have a plurality of lines of equipment, whereby the installation cost is further increased.
(c). In a method such as that of the above mentioned process, wherein the total quantity of the matte and the slag is fed into the second furnace, the load on the second furnace increases. For this reason, it cannot be said that the method is advantageous in all cases from the viewpoint of economy of equipment and heat. This point is particularly important in providing an improved method of smelting with a reverberatory furnace which is operated, not by a consistent, continuous process up to the copper producing process step as in the above mentioned process, but in combination with a final copper producing process such as that in a converter.